The metabolism of very low density lipoprotein-B (VLDL-B) peptide was studied in nineteen subjects with endogenous hypertriglyceridaemia (Types V, IV and IIb), three patients with heterozygous familial hyperbetalipoproteinaemia (Type IIa) and eight healthy subjects, by reinjecting autologous radioiodinated VLDL. The kinetics of VLDL-B peptide were followed. The mean turnover rate of VLDL-B peptide was significantly higher in the hypertriglyceridaemic group than in the control group but a considerable overlap in turnover rate was found between these groups. The patients with heterozygous familial hyperbetalipoproteinaemia had a normal turnover rate of VLDL-B peptide. A significant positive correlation was found between the turnover rate of VLDL-B peptide and VLDL-triglyceride concentration in the whole series. It is concluded that the underlying defect in endogenous hypertriglyceridaemia is heterogeneous. Overproduction of VLDL is a major determining factor in some patients whereas a reduced clearance is the determining factor in others.